Typically, an O2 sensor is mounted in the exhaust system of an engine to monitor how much unburned fuel or excess oxygen is in the exhaust as the exhaust gas exits the engine. Monitoring oxygen levels in the exhaust is a way of gauging the fuel mixture.
An O2 or lambda sensor is based on solid-state electrochemical reactions. The sensor can be constructed of one or two cells. When coupled to an appropriate electrical interface, a current is produced that varies approximately linearly with the partial pressure of oxygen in the exhaust gas relative to that in the atmosphere.
In wide-range air fuel sensors, the temperature of the sensor must be tightly controlled at temperatures of around 750 C. This temperature is achieved by use of a resistive ceramic heating element within the sensor. Typically, cell impedance measurements are performed on an O2 Sensor in order to estimate its temperature. This temperature is subtracted from a desired temperature to provide an error value. This error term can be used as an input to a control loop that varies the PWM duty cycle of battery voltage applied across an e.g. ceramic heating element embedded in the sensor. This control loop seeks to maintain the temperature error at a low value.
So in other words, as mentioned, in wide-range air fuel sensors, the temperature of the sensor must be tightly controlled at temperatures of around 750 C. This temperature is achieved by use of a resistive ceramic heating element within the sensor. This element is capable of heating the sensor from ambient alone or by supplementing the engine exhaust heat. In order to provide power to the resistive ceramic heating element, the automotive 12V battery voltage is applied across the element in a pulse width modulated fashion. The impedance of the sensor reference cell is measured and used to indicate the temperature of the sensor. A control loop is utilized to modify the PWM duty cycle to adjust the amount of heating to maintain the desired temperature.
In the control of O2 sensors, a problem exists where the PWM control of the ceramic heater can disrupt the closed loop control of the reference and pump cells of the sensor. This results in the PWM switching edges causing pump cell current errors for an extended period of time after the edge. This limits the ability to use the sensor where engine synchronous pump current readings are desired. Since the O2 sensor is located in the exhaust stream, there is an electrical cable that connects it to the sensing module. For a typical two cell sensor, this cable contains six wires which include: reference, sensor common, pump, tag, heater+ and heater−. Due to the nature of the heating element, the PWM control of the sensor's heater causes large DV/DT's and large DI/DT's in the heater wires within the cable as well as within the sensor itself. The current in the heater can be on the order of a few amps. This can couple noise onto the other wires in the cable.
It is one object of the invention to overcome these problems.